Crystalline stereocomplexes of highly stereoregular isotactic poly(methyl methacrylate) (it-PMMA) and syndiotacite poly(methyl methacrylate! (st-PMMA) blends formed by treatment with supercritical CO2 were investigated by means of differential scanning calorimetry (DSC) measurements as a function of PMMA molecular weight. The rates of stereocomplex formation obtained with supercritical CO2 treatment were faster than those obtained by thermal treatment. The higher rates of stereocomplex formation can be attributed to the plasticizing effect of CO2 molecules in the PMMA. The heat of melting of the stereocomplex (DeltaH) decreased with increasing PMMA molecular weight and was found to be independent of it-PMMA and st-PMMA mixing ratio. For the case of it-PMMA/st-PMMA of 1:1, the stereocomplexes obtained by thermal and CO2 treatment had two endothermic peaks. The relative intensity of the second endothermic peak ((DeltaH(3)/DeltaH) decreased with increasing PMMA molecular weight. and showed stronger dependence on it-PMMA molecular weight than on the st-PMMA molecular weight.